For what can be known about God is plain to them. To whom?

Roman 1:

18 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who by their unrighteousness suppress the truth.

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Paul was targeting his remarks at unrighteous people. At that time, there were no physicists and not many atheists around Rome. The unrighteous people were pagans.

19 For what can be known about God is plain to them, because God has shown it to them. 20 For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse.

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It was obvious to the pagans that God created the world. In fact, the pagans knew god/God already,

21 For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. 22Claiming to be wise, they became fools, 23and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and creeping things.

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The pagans worshipped idols.

Roman 1 is not about proving God's existence to scientists and atheists. Paul targeted the Roman pagans who knew god already but practiced idolatry. They worshipped the creation instead of the creator. You know there is a god; what kind of god will you worship?